1. Field of the Invention
This invention relates to an injection molding system for making a hollow object, and more particularly to an injection molding system for making a preform having an undercut.
2. Description of the Related Art
With a conventional injection molding system which performs injection molding of a preform, and performs stretching and blowing of a hollow object in one process, a preform injection-molding system comprises a neck mold for defining an outer wall of a neck, a cavity mold for defining an outer wall of the other part of the preform and a core mold for defining an inner wall of the preform. The core mold is positioned in the neck and cavity molds so that the preform can be made by introducing a plastic molding compound into the space between these molds. The molds are opened and clamped by forward and backward movement of one mold clamping unit. Specifically, the core and cavity molds are vertically moved for this purpose. Such injection molding systems are disclosed in a number of publications.
The preform sometimes has an undercut which is reversely tapered at a lower portion of the neck of the preform. In such a case, the molds cannot be opened only by moving the cavity mold downwardly. Therefore, the cavity mold includes two mold halves, which have to be horizontally opened and clamped.
Two mold clamping units should be used to move the molds vertically and to move the two mold halves horizontally. Since the mold clamping unit is required to exert a relatively large clamping force, addition of another mold clamping unit inevitably enlarges and complicates the injection molding system. Two independent energy sources also have to be installed to operate the two mold clamping units.